This invention relates to a method of forming crystallite-oriented films, thin films or other like deposits of superconducting ceramics and the crystallite-oriented superconducting ceramic films made by the method. More particularly, the method involves electrodeposition of a mixture of metals of the type and in a proportion sufficient to be oxidized into a superconducting ceramic, followed by oxidizing the electrodeposited mixture of metals to form the superconducting ceramic film. Crystallite orientation is achieved in the electrodeposition or oxidation steps, or by means of a separate treatment of the electrochemically deposited metal layer or of the ceramic film.
Superconducting materials have been known since 1911. However, the synthesis of superconductors having relatively high transition temperatures above 30.degree. K. is a quite recent development. By superconductors we herein mean such high transition temperature superconductors.
One class of these superconductors has been found to be superconducting near 90.degree. K. and has been identified as an oxygen deficient perovskite corresponding to the general composition MBa.sub.2 Cu.sub.3 O.sub.y (referred to hereinafter as the 1-2-3 material), where M is La, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Th or combinations of these elements. Two sub-classes of the 1-2-3 materials are: (a) an oxygen-reduced form, with an oxygen content of about y .TM.6.7, which has a transition temperature (Tc) of about 60 K, and (b) a doped form referred to sometimes as the 3-3-6 structure of general formula M(Ba.sub.2-x M.sub.x)Cu.sub.3 O.sub.7+.delta. in which M=Y, La, Sm, Eu, Gd, Tb, Dy, Ho. Er, Tm, Yb, Lu or Th, where T.sub.c ranges from 0 to about 60 K depending on x and annealing conditions. A second independent class with a Tc of between 20 and 40 K consists of perovskite materials of composition corresponding to La.sub.2-x M.sub.x CuO.sub.4, where M is Sr, Ba or Ca. These materials have been characterized by a variety of techniques (Extended Abstracts of the Materials Research Society Spring Meeting, Anaheim, California, 1987 and "High Temperature Superconductors", Materials Research Society Symposium Proceedings, Vol. 99 (1988)). More recently Bi and Tl containing compositions and phases such as Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8 and Tl.sub.2 Ba.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8, superconducting near 90K and 108K respectively, and a Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, phase superconducting near 127K, (Proceedings of Conference on Materials and Mechanisms of High T.sub.c Superconductivity, Interlaken, Switzerland, 1988, to be published in Physica B.) have been identified.
Crystallite orientation of high T.sub.c ceramic superconductors increases the critical current density (Applied Physics Letters 52, 1179 (1988)). High current densities are required for applications such as superconducting motors, generators, and magnets. Much higher critical current densities (10.sup.6 A cm.sup.-2 at 77K for YBa.sub.2 Cu.sub.3 O.sub.x) are obtained (Materials Research Society Proceedings 99, 119 (1988)) for crystallite-oriented films than are obtained for the corresponding crystallite-unoriented bulk superconductor at the same temperature (10-10.sup.3 A cm.sup.-2). Crystallite orientation can also be important for improving the mechanical and thermal durability of the superconducting films, which are typically subjected to both large temperature changes and high mechanical stresses during applications.
Prior art methods of manufacturing superconducting compositions involve mixing together amounts of compounds having the desired metals in ratios as they are found in superconducting compounds, treating the resultant mixture in a complex series of steps, followed by firing in an oven to oxidatively form the superconducting ceramic composition (Extended Abstracts of the Materials Research Society Symposium, Anaheim, Calif., 1987). The resultant products are typically in powder form and, thus, are not readily usable in practical applications.
Other exemplary prior art methods of making the superconducting ceramics involve forming a melt of the metal mixture and firing it in an oxidizing atmosphere: and the depositing an organometallic precursor from solutions followed by a firing in an oxidizing atmosphere ("High Temperature Superconductors", Materials Research Society, Symposium Proceedings, Vol. 99 (1988)). Processes in which the superconductor results from direct oxidation of salt or organometallic precursors have the disadvantage of potential incorporation of elements, such as carbon, from the precursors, which can degrade superconducting properties.
A further prior art method for making ceramic superconductors such as the 1-2-3 phase in a useable form involves chemical vapor deposition of the metals, followed by oxidation of the deposit into a ceramic film. This technique however, is complicated, and precision deposition on desired areas or on desired paths has not yet been achieved. Moreover, the deposition technique is complicated, requires high vacuum and high deposition temperatures. ("Thin Film Processing and Characterization of High Temperature Superconductors", No. 165, American Vacuum Society Series, editors J. M. E. Harper, R. J. Colton and L. C. Feldman, 1988). These complications are avoided by the herein described method.
Electrochemical techniques have heretofore been used to electrochemically vary the oxygen content of certain high temperature superconductors, ("High Temperature Superconductors", Materials Research Society, Symposium Proceedings, Vol. 99 (1988)). We are not aware of any prior art for electrochemically forming combinations of metals that are precursors to high temperature superconductors, nor of electrochemical formation of combinations of metals similar to those found in high temperature superconductors. We have found no disclosure of methods for making crystallite oriented films of ceramic superconductors based on electrochemically deposited metal precursors.
In particular, we are not aware of any precedent for codeposition of metals whose deposition potentials differ by as much as 3 V and, therefore, whose deposition rates and characteristics would be expected to differ substantially.
Those of ordinary skill in this art would not codeposit the combinations of metals found in high temperature ceramic superconductors by conventional electrodeposition methods because such combinations comprise one or more metals whose deposition from an electrolyte requires a highly cathodic potential. The aqueous electrolytes conventionally used in electrodeposition are reactive with materials having such highly cathodic reduction potentials at these potential levels. Thus, one would expect that such metals having highly negative reduction potentials would not effectively deposit on the substrate. By cathodic potential is meant a potential which allows electrons to be liberated, e.g., from an electrode to reduce the charge of a species in an electrolyte. By highly reducing potential is meant that which is substantially negative of the potential at which H.sup.+ is reduced to 1/2 H.sub.2 as at a normal hydrogen electrode (NHE). For example, each presently known precursor combination includes one or more metals that can be deposited only at potentials of more than 2V cathodic (negative) of NHE (e.g., Ca..sup.+2 at potentials &lt;-2.76 V vs normal hydrogen electrode, Sr..sup.+2 at &lt;-2.89 V, Ba.sup.+2 at &lt;-2.90 V, Y.sup.+3 at &lt;-2.37V). For comparison, copper, which is typically also required for formation of the high transition temperature superconductors, has a much more positive reduction potential for Cu.sup.+2 of +0.34 eV.